An article by our Psychology staff at ACS

There are many things that influence the development of a child. This chapter considers the influence of “nature” – the hereditary aspects. A child’s DNA will determine their eye colour, physical build, height and other physical characteristics. DNA can also have a role in determining character traits such as personality, aggression, and intelligence. 

The other major determinant of child development is "nurture," or the environmental influences. Within psychology there has been an ongoing debate known as the nature versus nurture, or nature-nurture debate which probably reached its height in the 1950's and 1960's. This debate has been concerned with the extent to which heritability and environmental factors are each responsible for developmental characteristics. Whilst there was a time when many psychologists would adopt a particular polar stance, these days there are few who would claim that nature or nurture are responsible for human development at the exclusion of the other. Nature and nurture can be more accurately scrutinised by considering how much each contributes to, and in what way they each affect, development.  

GENETICS

Some of the early proponents of heritability were interested in intelligence. Francis Galton in the UK was a supporter of eugenics and founder of the eugenics movement in the 1880's. He claimed that intelligence was passed on in families and so must be inherited. Furthermore, members of the lower classes were less intelligent and were having far too many children - which would ultimately dilute the overall intelligence of the population. He went as far as to call for sterilisation of those of lower intelligence and his ideas were widely adopted for some time in the UK and the US.

Galton based his ideas on data he gathered from physical measurements of people such as circumference of the head, chest measurements, and height. From this he extrapolated that intelligence level was a result of physical characteristics. He argued that each person could be classified according to their intelligence level and that this was fixed from birth. From Galton's perspective, no matter how much time and effort a parent or teacher puts into helping a child to develop cognitively, they will never be able to exceed the upper limits of their inherited capacity for intelligence.

In the US in the early 1900's, Goddard took up Galton's ideas on the heritability of intelligence and published work on a 'degenerate' family he referred to as the 'Kalikaks.' This work was often used in an attempt to persuade others about how poor heredity could be passed on in society if such degenerate types were allowed to breed without restraint. He claimed that there was a single gene for feeble-mindedness. Thankfully, such extreme views are no longer widely held in society.    

Other proponents of the heritability of intelligence were Spearman and Burt. Spearman believed that general intelligence could be represented as a single score on an intelligence test - he referred to this as the "g factor." Cyril Burt's ideas on the heritability and fixed nature of intelligence resulted in the introduction of 11-plus exams in the UK in the 1940's which determined what type of education a child at age eleven would go on to have. Those who performed well went on to receive a more privileged grammar school education.

Many of Burt's assertions with regards to the heritability of intelligence were based on his studies of twins, which have since been found to be flawed (see problems below).

TWIN STUDIES AND HERITABILITY   

The study of identical (monozygotic) twins and non-identical (dizygotic) twins helps to determine the relative influence of nature (heritability) and nurture (environment). Because identical twins are genetically identical, it is assumed that any differences between them have been created by the environment. Heritability is thought to be responsible for characteristics that are more similar in identical than non-identical twins who have been reared in the same environment.   

Less commonly, identical twins who have been separated at birth provide further evidence of the influence of nature on development. Identical twins separated at birth allow for observations of people with identical genes (identical by nature), reared in different environments (different by nurture). These studies often show extraordinary similarities between the twins that have been reared apart - in personality, attitudes, mannerisms, abilities, intelligence, and even their choice of partners and careers.  However, we can only conclude that these similarities are probably due to nature. 

Twin studies are important in studying the significance of genetics and the environment on any factors affecting us, so let’s consider these in more detail.  

Psychology experiments involve controlling one factor so as to determine its effect on something else. With twin studies, psychologists are able to use a natural form of experimental design because there is no need for a control (non-experimental) group.  Francis Galton, the pioneer of twin research, referred to twins as “naturally occurring experimental design” and as a “living laboratory.”

With twins, we can compare how alike they are. For example, research has shown that when one identical twin has autism, in 60% of cases their twin also has autism. This suggests a strong genetic link. Twin studies do not just tell us about genes though, they also tell us about the importance of the environment. If the results were the other way round i.e. in 60% of cases if one twin has autism the other does not, this would suggest more of an environmental factor. 

PROBLEMS WITH TWIN STUDIES

Twin studies are not infallible though. Whilst they are a natural form of experimentation, in order to get a good sample hundreds of pairs of twins need to be studied.  Cyril Burt who used his twin studies to convince others of the heritability of intelligence was later accused by his biographer, Hearnshaw, of claiming to have tested far more pairs of twins (53) than he actually did, because he only kept records of 21 pairs of twins.

Other difficulties relate to the fact that much of the data derived from twin studies can only be considered probable at best. Some studies have looked at similarities - if the similarity in behaviours between individuals is related to the degree of genetic similarity between them, then obviously we would expect identical twins to have more similar behaviours, followed by fraternal twins, siblings, grandchildren and so on. Therefore, if there is a genetic basis for say, alcoholism - we would expect a higher probability that if one identical twin is an alcoholic that his or her twin will be, and the likelihood of alcoholism would become gradually diluted as the degree of genetic similarity becomes less. However, there are many similarities between relatives which are not genetic. For example, they may follow the same sports or support the same teams but you couldn't argue there was a genetic basis for this. In fact, comparisons of similarity amongst relatives are usually used to disprove a genetic basis for a particular condition.

Data from identical twin studies where the twins have been reared in the same environment is also flawed. Since these twins have more genetics in common than fraternal twins they are more likely to behave similarly and this might be interpreted as indicative of genetics. However, the fact that they behave similarly also suggests that they are likely to be treated similarly by their parents and others and so they share greater environmental (nurture) similarity. Even with the hardest evidence from studies involving identical twins reared apart we still cannot rule out the possible influence of the prenatal environment on behaviour.     

Although twin studies have fallen in and out of fashion, but they are used increasingly in research areas like gene technology to look at the bases for complex diseases such as diabetes, cancer, heart disease, and so forth.  

We shall consider twin studies and the influence of nurture in the next chapter.

ADOPTION STUDIES

Another way of looking at the effects of nature on behaviour is through adoption studies. Evidence from sibling adoption studies is less compelling than twin studies since identical twins have identical genetics whereas siblings only share half their DNA. 

If a child is adopted very young and reared in a different environment by different parents to those who provided its genes, it has been shown that in some cases the likelihood of some behaviours developing corresponds more highly with whether the biological parents also express that behaviour rather than the adoptive parents. This would seem to support the argument for a genetic basis for behaviour, unless the prenatal environment caused such influences.

Perhaps more convincing is evidence from half-sibling studies. Some studies have been conducted involving paternal half-siblings (siblings with the same father but a different mother and therefore a different prenatal environment), who were adopted when young into different families and different environments. In these circumstances, behaviours which are found to be similar would most likely be due to genetics and not to chance.

THE INTERACTION OF NATURE AND NURTURE

So, how do our genes affect our behaviour? These days, most psychologists believe that our behaviour is derived from a mixture of the influences of both nature and nurture. We may have a gene for aggression, for example, but only in certain conditions will that gene be “turned on” and we develop inappropriate aggression. Breast cancer is also thought to have a genetic input, but again, that gene is only “turned on” in certain environmental conditions, suggesting an interaction between nature and nurture. 

In fact, the whole nature versus nurture debate is probably quite pointless because it fails to acknowledge how we develop from a biological perspective. We cannot assume that development is caused by either nature or nurture. Essentially, we are born with a genetic blueprint or genotype which is a pattern of genes passed on to us by our parents. Our interactions with the environment then determine to what extent certain characteristics from the genotype are expressed and this results in what is known as our phenotype. In other words, the genotype might be a fixed - but the phenotype is not. The phenotype is what a child or adult becomes.   

Consider a child who is genetically physically attractive. This is their genotype. Studies have shown that physically attractive children are treated more positively than less attractive children at school. This can give a child a greater sense of self-worth. They are also likely to get more attention and so more chance to develop academically. This in turn leads to greater confidence. The way the child is treated (an aspect of nurture) occurs in response to their physical appearance (an aspect of nature i.e. it is influenced by their genotype), and so their resulting personality, behaviours, intelligence, etc. are created by a combination of nature and nurture. 

In the above example, we could say that the child's genotype has had a positive influence on their phenotype. In other words the interaction of heritability and experience has produced a confident child who is likely to be academically successful and who has a great sense of self-worth. If the same child was abused or bullied because of their looks or grew up in a society where they were not valued for their appearance then a very different child could emerge - one who lacks confidence, who is withdrawn and fearful, and lacking in trust of others.  

Clearly then, nature and nurture both play a role in development, and it is the interaction of the two which is of most significance. We can also see that whilst genes do play a role in development their role is not a fixed one because different genes are expressed at different times throughout a child's development in accordance with their interaction with the environment. The phenotype is constantly developing. What this all means is that if we take intelligence as an example - it is not a fixed characteristic. A child may attain a particular score on a particular test at a particular point in time, but they may attain a quite different score at another point in time depending on their experiences.